Allow plugins to register gmg commands (code review required)

[Ben Sturmfels]

Hi Folks,

I'm currently working on a project-specific variant of the batchaddmedia
gmg command that's not useful to have in core MediaGoblin. I'd like to
put the command in a plugin, but currently plugins can't register gmg
commands.

I've added the ability to define gmg commands in plugins in the
following branch. Does this approach make sense? Have I missed anything?

https://gitlab.com/BenSturmfels/mediagoblin/merge_requests/1/diffs

If this looks sound, I'll also update the pluginwriters docs.

Regards,
Ben

Ben Sturmfels

Sturm Software Engineering
www.sturm.com.au
+61 3 9024 2467